Typical processes known today for preparation of compound metal oxides are:
(1) A process comprising physically pulverizing and mixing oxide, carbonate or oxalate of respective metals, firing the mixture and pulverizing the fired mixture; and
(2) a process comprising preparing a homogeneous aqueous solution of a water-soluble salt such as nitrate, hydrochloride, etc. of respective metals, adding hydroxide ions, carbonate ions, oxalate ions or the like to the solution to form insoluble coprecipitate of hydroxides, carbonates, oxalates or the like of the metals, collecting the precipitate and firing it.
In the process (1), mixed powder is fired. Therefore, the reaction proceeds from the boundary of the particles and is liable to leave unreacted portions. This means that the products are not uniform and the composition thereof deviates from the intended one.
In the process (2), conditions for precipitation differ from element to element in a solution. Therefore, from a solution containing a plurality of metals in a specific content ratio, a compound metal oxide in which the metals are contained in the same content ratio as in the solution cannot be formed.
We have found that compound metal oxides of bismuth, strontium, calcium and copper of specific compositions can be prepared by spraying an organic solvent solution containing a bismuth alkoxide, a strontium alkoxide, a calcium alkoxide and an alkoxy-.beta.-diketo complex, .beta.-diketo complex or .beta.-ketoester complex of copper in an intended specific content ratio with respect to the metals and firing the sprayed mist after the solution has been allowed to react.